1. Field of the Invention
This invention relates to improvements of a high-pressure tank into which high-pressure gas is charged and improvements of a method for fabricating the same.
2. Description of the Related Art
High-pressure tanks into which a gas, such as natural gas or hydrogen gas, is charged and stored at high pressure, are generally subjected to winding which is a technique for wrapping carbon fibers or the like around the tank body for reinforcement. A cylindrical gas discharge section and a continuous dome section of the tank, in particular, are likely to concentrate stress and therefore must be reinforced firmly. However, the vicinity of the boundary between the dome section and the cylindrical gas discharge section is difficult to subject to winding and thus difficult to reinforce.
Therefore, the dome section and the cylindrical gas discharge section are generally increased in thickness as compared with a cylindrical middle section of the tank by necking a predetermined region of an elongated hollow cylindrical blank continuing from its opening end by means of spinning.
In this connection, the applicant filed a patent application on a technique for further thickening the dome section and the cylindrical gas discharge section by flow forming the predetermined region of the elongated hollow cylindrical blank continuing from its opening end into a larger thickness than the other region and then necking the thickened predetermined region through spinning, and has already obtained a patent for the technique (see, for example, Patent document 1)
[Patent Document 1]
Japanese Patent Publication No. 3251216 (page 3 and FIG. 1)
For the first-mentioned technique, however, there is a limit to how much the thickness of the dome section and the cylindrical gas discharge section can be increased because of the use of an elongated hollow cylindrical blank having a uniform thickness as a whole. Therefore, this technique is difficult to apply particularly to high-pressure tanks for hydrogen gas into which the gas is charged at a pressure of between 35 MPa and 75 MPa.
To cope with this, it can be considered to ensure the thickness of the dome section and the cylindrical gas discharge section by increasing the thickness of the entire middle section of the elongated hollow cylindrical blank. In this case, however, the entire high-pressure tank thus fabricated will also be thickened and thereby increased in weight, which hinders weight reduction. In addition, since the tank uses the thick elongated hollow cylindrical blank, its material cost will be high.
On the other hand, for the second-mentioned Patent document 1, since the predetermined region of the elongated hollow cylindrical blank continuing from the opening end is increased in thickness as compared with the other region, the dome section and the cylindrical gas discharge section can have a larger thickness as compared with the first-mentioned technique and the entire tank weight can be reduced by thinning the middle section. This Patent document 1 technique, however, requires the elongated hollow cylindrical blank to undergo two process steps of flow forming and spinning, which takes much labor. In addition, in order to ensure a sufficient thickness of a dome section and a cylindrical gas discharge section in a high-pressure tank for hydrogen gas, it is necessary to increase the amount of reduction of the blank. This may invite the complete closure of the cylindrical gas discharge section and thereby necessitate subsequent boring of a gas outlet in a later process step, which requires extra labor.